Resolve "Replace inspect.getargspec references"

Closes #208 (closed)

bilby/core/prior.py

@@ -424,8 +424,7 @@ class Prior(object):
         str: A string representation of this instance
 
         """
-        subclass_args = inspect.getargspec(self.__init__).args
-        subclass_args.pop(0)
+        subclass_args = [k for k in inspect.signature(self.__init__).parameters.keys()]
         prior_name = self.__class__.__name__
 
         property_names = [p for p in dir(self.__class__) if isinstance(getattr(self.__class__, p), property)]